1. Field of the Invention
The present invention relates generally to the field of implantable vibrational transducers, such as those used for correcting hearing impairment, and more particularly, to methods for fabricating such transducers using microfabrication processes and transducers produced by such processes.
The seemingly simple act of hearing is a thing that can easily be taken for granted. Although it seems to us as humans we exert no effort to hear the sounds around us, from a physiologic standpoint, hearing is an awesome undertaking. The hearing mechanism is a complex system of levers, membranes, fluid reservoirs, neurons and hair cells which must is all work together in order to deliver nervous stimuli to the brain where this information is compiled into the higher level perception we think of as sound.
As the human hearing system encompasses a complicated mix of acoustic, mechanical and neurological systems, there is ample opportunity for something to go wrong. Unfortunately this is often the case. It is estimated that one out of every ten people suffer some form of hearing loss. Surprisingly, many patients who suffer from hearing loss take no action in the form of treatment for the condition. In many ways hearing is becoming more important as the pace of life and decision making increases as we move toward an information based society. Unfortunately for the hearing impaired, success in many professional and social situations may be becoming more dependent on effective hearing.
A number of auditory system defects are known to impair or prevent hearing. To illustrate such defects, a schematic representation of part of the human auditory system is shown in FIG. 1. The auditory system is generally comprised of an external ear AA, a middle ear JJ, and an internal ear FF. The external ear AA includes the ear canal BB and the tympanic membrane CC, and the internal ear FF includes an oval window EE and a vestibule GG which is a passageway to the cochlea (not shown). The middle ear JJ is positioned between the external ear and the middle ear, and includes an eustachian tube KK and three bones called ossicles DD. The three ossicles DD: the malleus LL, the incus MM, and the stapes HH, are positioned between and connected to the tympanic membrane CC and the oval window EE.
In a person with normal hearing, sound enters the external ear AA where it is slightly amplified by the resonant characteristics of the ear canal BB. The sound waves produce vibrations in the tympanic membrane CC, part of the external ear that is positioned at the distal end of the ear canal BB. The force of these vibrations is magnified by the ossicles DD.
Upon vibration of the ossicles DD, the oval window EE, which is part of the internal ear FF, conducts the vibrations to cochlear fluid (not shown) in the inner ear FF thereby stimulating receptor cells, or hairs, within the cochlea (not shown). Vibrations in the cochlear fluid also conduct vibrations to the round window (not shown). In response to the stimulation, the hairs generate an electrochemical signal which is delivered to the brain via one of the cranial nerves and which causes the brain to perceive sound.
The vibratory structures of the ear include the tympanic membrane, ossicles (malleus, incus, and stapes), oval window, round window, and cochlea. Each of the vibratory structures of the ear vibrates to some degree when a person with normal hearing hears sound waves. However, hearing loss in a person may be evidenced by one or more vibratory structures vibrating less than normal or not at all.
Some patients with hearing loss have ossicles that lack the resiliency necessary to increase the force of vibrations to a level that will adequately stimulate the receptor cells in the cochlea. Other patients have ossicles that are broken, and which therefore do not conduct sound vibrations to the oval window.
The hearing impaired patient today has a wide variety of hearing devices to choose from. Devices that have improved circuits, enhanced fitting parameters that allow the electronics to be customized to the patients individual hearing loss (i.e., similar to an eye glass prescription, one size does not fit all). New devices located completely in the patients ear canal are available that are cosmetically superior to the large bulky devices of years past and can be virtually invisible. Many manufacturers participate in the hearing marketplace which is a sizable 3 billion dollar worldwide market.
Prostheses for ossicular reconstruction are sometimes implanted in patients who have partially or completely broken ossicles. These prostheses are designed to fit snugly between the tympanic membrane CC and the oval window EE or stapes HH. The close fit holds the implants in place, although gelfoam is sometimes packed into the middle ear to guard against loosening. Two basic forms are available: total ossicular replacement prostheses which are connected between the tympanic membrane CC and the oval window EE; and partial ossicular replacement prostheses which are positioned between the tympanic membrane and the stapes HH. Although these prostheses provide a mechanism by which vibrations may be conducted through the middle ear to the oval window of the inner ear, additional devices are frequently necessary to ensure that vibrations are delivered to the inner ear with sufficient force to produce high quality sound perception.
Various types of hearing aids have been developed to restore or improve hearing for the hearing impaired. With conventional hearing aids, sound is detected by a microphone, amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or another type of transducer into the middle ear by way of the tympanic membrane. Often the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion.
A recent breakthrough in the field of hearing impairment treatment is described in U.S. Pat. Nos. 5,554,096; 5,559,358; and 5,624,376, each of which is assigned to the assignee of the present application. These patents describe the use of floating mass transducers which may be implanted or mounted externally for producing vibrations in the vibratory structures of the ear. The floating mass transducers include a housing which may be vibrationally coupled to the vibratory structure within the ear, a mass mechanically coupled to the housing, and some means for vibrating the mass in response to an externally generated signal. Vibration of the mass, in turn, causes inertial vibration of the housing to produce the desired vibrations in the vibratory structure of the ear. The use of floating mass transducers for stimulating the vibratory structures of the ear has a number of advantages over other implantable devices. In particular, floating mass transducers can produce vibrations in the cochlea that have sufficient force to stimulate hearing perception with minimal distortion.
As described in these three patents, and in the co-pending applications which are listed below and also assigned to the assignee of the present application, the floating mass transducers are small devices composed of a number of discreet components which are attached to each other by conventional techniques, usually by hand construction. A first generation device being produced by the assignee of the present invention comprises a housing, and a magnet, a spring mechanism carrying the magnet, and a magnetic coil within the housing. Each of these components is placed in a metal housing, typically by hand, and the housing sealed and electrical leads brought out of the housing, also by hand. The method of fabrication is time-consuming, expensive, and presents a chance of human error in constructing the device. For these reasons, it would be desirable to provide improved fabrication methods for producing floating mass transducers for use in implantable devices for the treatment of hearing impairment and other purposes. It would be particularly desirable if such fabrication methods could be performed with little or no direct manual manipulation of the transducer components. It would be still further desirable if multiple copies of the transducers could be fabricated simultaneously. It would be even more desirable if such multiple transducers could be produced with highly accurate and repeatable specifications and tolerances. At least some of these objectives will be met by various aspects of the present invention, as described below.
2. Description of the Background Art
Micromachined transducers for the measurement of pressure and force are described in U.S. Pat. Nos. 5,559,358; 5,553,506; 5,473,944; 5,458,000; 5,338,929; and 5,165,289. Implantable vibratory transducers for mounting on the vibrating structures of the middle ear are described in U.S. Pat. Nos. 5,456,654; 5,554,096; and 5,624,376, assigned to the assignee of the present application. U.S. Pat. No. 5,624,376 describes an angular momentum mass magnet (FIG. 12) and a piezoelectric floating mass transducer (FIG. 13), each of which employ vibrating members which are anchored at one end. The following U.S. Patents contain related subject matter: U.S, Pat. Nos. 5,857,958; 5,949,895; 5,913,815; 5,800,336; 6,024,717; 5,795,287; 5,859,916; and 5,897,486. The full disclosures of each of these patents and pending applications are incorporated herein by reference.
The present invention provides unique vibrational transducers and methods for their fabrication. The vibrational transducers are miniaturized, typically having a maximum dimension below 3 mm, preferably below 2 mm, and are particularly suitable for medical uses. An exemplary medical use comprises direct or indirect attachment to a vibratory structure of an ear for the treatment of hearing impairment in human patients. Other medical uses include the transfer of sound energy to tissue surfaces and/or drugs for the enhancement of drug delivery, and the like. The vibrational transducers of the present invention will also be useful for non-medical uses, such as electrostatic converters.
The vibratory transducers of the present invention are particularly suitable for fabrication by micromachining or photolithographic techniques. Surprisingly, despite the design constraints resulting from the use of such fabrication techniques, the transducers of the present invention may be miniaturized, typically having a maximum dimension below those set forth above, while possessing the internal components necessary to produce a minimum displacement of at least 0.1 xcexcm, preferably at least 0.5 xcexcm, more preferably at least 1 xcexcm, and even more preferably at least 3 xcexcm, with useful ranges from 0.1 xcexcm to 10 xcexcm, usually from 1 xcexcm to 5 xcexcm, with a force sufficient to stimulate the vibratory structures within the ear to produce the perception of hearing in the patient.
In a first embodiment of the apparatus of the present invention, the transducers comprises a housing having an anchor region and a vibratory void therein. A vibrating member has a fixed end secured to the anchor region within the housing and a moving end which carries an inertial mass in the vibratory void within the housing. Means are provided for vibrating the moving end of the vibrating member in response to an applied electric signal so that vibrational force is transmitted to the anchor region of the housing. Usually, the housing will be sealed and be proportioned and adapted to be mounted on a vibratory structure of an ear for the treatment of hearing impairment. In such cases, a connector will usually be provided on the housing for attaching the housing to an ossicle of the middle ear or other vibratory structure. Optionally, a biocompatible coating may be provided over the exterior surface of the housing.
As described in more detail below, all portions of the vibratory transducer will usually be fabricated by photolithographic deposition, patterning, and etching processes. Thus, the vibrating member will be formed by a deposition process onto the anchor region of the housing. More usually, the housing will comprise a substrate, typically a silicon wafer, and the vibratory member will be formed by depositing a suitable bendable material, such as silicon nitride, thereover. Other components of the transducer will also be formed by deposition of the materials into the desired pattern, although it would also be possible to introduce some discrete components (i.e. components prepared separately from the photolithographic fabrication process) into the structure of the transducer.
Preferably, the vibrating member will comprise a beam having a hinge region to define a bending point. Usually, the hinge region will comprise a weakened region to define a solid hinge. In such cases, silicon nitride or other high Q materials are particularly preferred material for the beam since it can withstand a virtually unlimited number of vibrational cycles without material failure. It will also be possible, however, to form a mechanical hinge in the beam by surface micromachining techniques. The mechanical hinge would have the advantage of a very low natural frequency. The mechanical hinge, however, is generally more complicated to fabricate. Even when using a solid hinge, such as a thin region within the beam, it is desirable that the vibrating member in combination with the inertial mass have a natural frequency below 2000 Hz, preferably below 1500 Hz. In this way, the resonant characteristics of the beam will not interfere with vibrating the beam within the desired acoustic range, typically from 200 Hz to 10,000 Hz, when the transducer is used for treatment of hearing impairment.
The means for vibrating the vibrating member typically comprises an electrically conductive element on the vibrating member and at least one electrically conductive element fixed relative to the housing. By applying an electrical signal between the two electrically conductive elements, the vibrating member can be driven to vibrate the inertial mass and produce the desired inertial vibratory force. Usually, the vibrating means will further comprise a second electrically conductive element fixed relative to the housing. In the preferred embodiment, the two electrically conductive elements which are fixed relative to the housing are aligned over and under the electrically conductive element on the vibrating member. The upper and lower electrically conductive members can then be alternately driven in order to produce the desired vibratory pattern in the vibrating member. When using only a single electrically conductive pad fixed relative to the housing, the cantilever will have to possess a sufficient spring force to allow it to react after it has been driven. Such spring-loaded cantilever will generally possess much poorer frequency response than will comparable systems where the hinge has a very low spring force when driven.
In a second aspect of the apparatus of the present invention, an implantable vibrational transducer comprises a sealed housing having a generally planar base structure defining an anchor region and a vibrational void adjacent to the anchor region. A cantilever beam is mounted within the housing and has a fixed end secured to the anchor region and a free end which typically extends to or over the vibrational void. An inertial mass is secured to the free end of the cantilever beam and is disposed to vibrate in the vibrational void. An electrically conductive element is formed on the cantilever beam between the fixed end and the free end, and at least a first electrically conductive drive element is fixed relative to the housing and disposed adjacent to and spaced-apart from one side of the electrically conductive element on the beam. Thus, by applying an electrical signal between the drive element and the beam element, the beam will cause to vibrate the inertial mass and transmit an inertial force back to the housing. The housing of the implantable vibrational transducer is preferably proportioned and adapted to be mounted on a vibratory structure of the ear, and usually comprises a connector adapted for attaching to an ossicle or other vibratory structure. The housing may further comprise a biocompatible coating over its exterior surface, and the housing will typically be composed of at least in part of silicon and the cantilever beam will typically be composed at least in part of silicon nitride.
In a preferred embodiment of the implantable vibrational transducer, the cantilever beam is adapted to act as a spring having a natural frequency, in combination with the inertial mass, below 2000 Hz. The cantilever beam will typically comprise a weakened region to define a solid hinge, having the preferred natural frequency, alternatively, the cantilever beam could comprise a mechanical hinge having a very low natural frequency, typically approaching zero. In such cases, an electronic driver for the transducer (described in more detail below) can be adapted to provide electrical biasing of the free end of the cantilever beam in order to provide an xe2x80x9celectronic spring.xe2x80x9d In all cases, the inertial mass will typically have a mass in the range from 3 mg to 50 mg, preferably from 5 mg to 15 mg.
In a further preferred aspect of the implantable vibrational transducer, the vibrational void will be defined at least in part by a recess formed in the base structure. The inertial mass is preferably disposed at least partly within the recess, and a clearance between the inertial mass and a wall of the recess will typically be in the range from 3 xcexcm to 30 xcexcm, preferably from 3 xcexcm to 10 xcexcm, when the mass is at rest relative to the base structure. This clearance is between the adjacent surfaces which move relative to each other when the mass is vibrated. Thus, the clearance provides the necessary room for the mass to vibrate when it is being electrically driven.
In an exemplary embodiment of the implantable vibrational transducer, the cantilever beam is composed of silicon nitride, has a length in the range from 0.5 mm to 2.5 mm, preferably from 1 mm to 2 mm, a width in the range from 0.2 mm to 2 mm, preferably from 0.5 mm to 1 mm, and a thickness in the range from 0.01 mm to 0.5 mm, preferably from 0.01 mm to 0.1 mm. The inertial mass will have a mass in the range set forth above, with a volume in the range from 0.1 mm3 to 2 mm3, preferably from 0.1 mm3 to 0.5 mm3. Thus, the mass will typically be formed from a dense metal, such as gold, silver, platinum, or the like. The geometry of the implantable vibrational transducer may vary widely, and it may be in the form of a cylindrical solid, rectangular solid, triangular solid, or it may have an irregular configuration. Typically, at least one face will be flat since the device will normally be formed from a planar silicon wafer starting material, as described below. In the case of rectangular solids, the length will typically be in the range from 1 mm to 4 mm, preferably from 1.5 mm to 2.5 mm, the width will be in the range from 1.5 mm to 3 mm, preferably from 1.5 mm to 2 mm, and the thickness will be in the range from 1.5 mm to 2.5 mm, preferably from 1.5 mm to 2 mm.
The implantable vibrational transducers of the present invention will preferably be configured so that the inertial mass will have a maximum vibrational displacement in the range from 1 xcexcm to 100 xcexcm, preferably from 1 xcexcm to 30 xcexcm, when the mass is vibrated within the expected range, typically the auditory range from 200 Hz to 10,000 Hz. The cantilever beam will typically be driven with a power ranging from 10 xcexcwatts to 200 xcexcwatts, preferably from 10 xcexcwatts to 50 xcexcwatts. The vibrational energy will depend on the power at which it is being driven.
According to the present invention, a method for fabricating a vibrational transducer comprises providing a base structure, typically in the form of a silicon wafer or other conventional material suitable for microfabrication, e.g. photolithographic processing. A cantilever beam is formed on a surface of the base structure, typically by deposition, and is patterned into a desired geometry. The cantilever beam will be anchored at one end to the base structure and will have a free end which is capable of vibrating relative to the base structure. An inertial mass is formed at the free end of the cantilever beam, and the inertial mass is disposed to vibrate in a free space. An electrically conductive drive element is formed on the cantilever beam in the space between the anchored end and the free end. A fixed electrically conductive drive element is also formed on or over the base structure so that it is adjacent to and spaced-apart from the electrically conductive element on the cantilever beam. Preferably, a second electrically conductive drive element is also formed on or over the base structure, with the exemplary embodiment comprising a first drive element on the base structure and a second drive element formed on a bridge over the cantilever beam. A housing is then formed over the base structure to seal the transducer so that it is suitable for implantation or other uses.
Preferably, the fabrication method further comprises forming a recess in the base structure to define at least a portion of the free space in which the inertial mass vibrates. The inertial mass may be formed within the recess, typically by first forming a sacrificial layer, then depositing the inertial mass material into the recess (over the sacrificial layer) and subsequently removing the sacrificial layer to provide a clearance between the inertial mass and the wall of the recess within the base structure. Preferably, the clearance will be in the range from 1 xcexcm to 100 xcexcm, preferably from 1 xcexcm to 30 xcexcm, which is sufficient to permit the desired vibrational amplitude described above.
In a further preferred aspect of the fabrication method of the present invention, the cantilever beam is formed by depositing a layer which extends from an anchor region on the base structure over a bridge structure defined by a sacrificial layer, and then on to the previously formed inertial mass (which will typically be in the recess). Subsequently, the sacrificial layer is removed to leave clearance for the cantilever being to vibrate together with the inertial mass. Preferably, at least one aperture will be etched into the cantilever beam to define a weakened hinge region. Further preferably, the cantilever beam will comprise silicon nitride which is deposited and patterned to have the dimensions set forth above.
The electrically conductive drive elements are preferably formed by depositing and patterning electrically conductive material on the substrate below the electrically conductive element on the beam. A bridge is then formed over the cantilever beam, and a second electrically conductive pad deposited and patterned on the bridge. In this way, the preferred stack of electrically conductive pads described above may be formed, with both pads fixed relative to the base structure.